As offshore drilling operations progress into deeper waters, especially in depths of water greater than 1000 feet, many relatively simple suface operation become complex and costly. One frequent operational requirements is that of engaging a hydraulic stab sub receptacle with a probe for the propose of applying hydraulic flow and pressure to operate a function. The function can be a valve, blowout preventer, test port, or other such items.
Systems which provide guidelines or wire ropes from the surface vessel to the ocean flooor equipment provide a predetermined path to the equipment which can be easily followed by service systems and can be keyed off of to locate the appropriate receptacle.
In operations conducted with dynamically positioned vessels, there are no typically guidelines to direct equipment and service systems to the ocean floor. Dynamically positioned vessels are those which are held in place by the power of their propellers rather than by anchors. In this case, finding the general location of the equipment and keying into a specific area for service operations such as hydraualic stabs is typically done by an ROV. An ROV is a remotely operated vehicle which is small unmanned submarine equipped with sonar and television systems.
The hydraulic stab receptacles are characteristically designed to contain relatively high pressures such as 3000, 5000 or 10,000 p.s.i. The close fit fit between the bore of the hydraulic stab receptacles and the outer diameter of the hydraulic stabs required to prevent the extrusion of the seals provides the conventional disadvantage of requiring a close angular alignment to allow this installation. It is generally estimated that a stab needs to make an engagement to a depth equal to the diameter of the stab to prevent binding. For a typical 1.375" outer diameter stab and a 0.010" diametrical tolerance, this mens that the stab must be aligned with the receptacle within less than 1/2 degree.
This degree of alignment is difficult to control on devices which are guided down guidelines to fixed positions before attempting to make the stab. When a free swimming vehicle such as an ROV attempts to make the engagement the problems are even more difficult. The stab and receptacle are viewed with one or more televisions cameras on the ROV. An operator at the surface must watch the television screens and make the engagement with remote manipulator arms or vehicle movement. The ROV is typically moving and the cameras' fisheye lenses distort the view of the parts.
The task can be accomplished in this fashion. It may take a long time, but the stab can eventually be worked into the receptacle. On a recent installation in the Gulf of Mexico, an ROV service company attached a piece of relatively stiff wire rope to a hydraulic stab receptacle and the flexibility assisted in the stabbing process. Making the devices oversized enough to allow for a stepped engagement will assist in eliminating the binding at the expense of size on each of the receptacles. U.S. Pat. No. 4,682,913 gives an illustration of this style design.
In shallower waters where divers are required engage hydraulic stab subs into receptacles, the close alignment is still a problem, but to a lesser degree. The diver is capable of wiggling the stab into alignment by hand much more readily than can be done by remote operations.